Sound signal system



Oct. 26, 1965 R. J. DODGE 3,214,753

SOUND SIGNAL SYSTEM v Habe/f L/ 50c/y@ INVENTOR Oct. 26, 1965 R, 1 DODGE SOUND SIGNAL SYSTEM 4 Sheets-Sheet 2 Filed May 22, 1962 oct. 26, 1965 Filed May 22, 1962 R. J. DODGE 3,214,753

SOUND SIGNAL SYSTEM 4 Sheets-Sheet 3 ffoef" d oa/g@ INVENTOR Oct. 26, 1965 R. J. DODGE SOUND SIGNAL SYSTEM 4 Sheets-Sheet 4 Filed May 22, 1962 V0 LTA GE REGUL ATOR Z po wE/z STAGE Q d /a e 5 4 A# @m e m J E l U d d n 7 mi /f w f m W/V/ w N ,o w O M] H j D12/VER STAGE (F RE QUE NC Y G ENE RATGR United States Patent O 3,214,753 SOUND SIGNAL SYSTEM Robert I. Dodge, Houston, Tex., assignor to Automatic Power, Inc., Houston, Tex. Filed May 22, 1962, Ser. No. 196,725 7 Claims. (Cl. 340-384) This invention relates to improved sound signal systems. It is particularly useful as a marine sound warning system or fog horn in unmanned, offshore locations,

such as platforms of offshore oil wells, but is not limited to such use.

Sound warning signals have been used for centuries to warn mariners of dangerous obstructions such as rocks, shoals or offshore structures. Bells, whistles, gongs, air horns, sirens and even booming cannons have been used to warn of unseen hazards and guide mariners safely through fog. Although many modern ships are equipped with radar, the sound signal still remains the final warning of impending collision.

International and federal laws require the use of sound signals during periods of reduced visibility; and in accordance with this requirement, audible warning systems are maintained on offshore structures. The law prescribes that a warning signal be audible at a range of two nautical miles in all directions and that it be operated whenever visibility is less than five miles. The production of sounds of such intensity requires a considerable amount of energy and the provision of a reliable source of sufficient energy to produce a noise loud enough to warn approaching ships at such distances has proved a major problem in the offshore oil production industry. At present, it is customary to use small diesel engines to provide power for such signals.

On unmanned platforms, where there is no practical way to determine when the visibility is less than five miles, the engines and sound warning seignals run constantly. Since a single engine is not considered sufficiently reliable, it has been customary to use dual engine systems with an automatic switchover device which will automatically start the second engine when the lirst engine fails for any reason. These systems are necessarily complicated and require a great deal of periodic maintenance. Tons of fuel, water, lubricating oil and spare parts must be transported to remote platforms and the machines must be carefully serviced at frequent intervals t-o insure a high degree of reliability, Operation and maintenance of these machines add greatly to the expense of maintaining an offshore oil well in production.

Offshore platforms also are equipped with ashing navigational lights. The energy requirement of these lights is only a small fraction of that required for the sound signal so that the lights can be powered by expendable primary batteries. Extremely dependable batteries of this type have been developed for such service by the U.S. Coast Guard. Batteries of Coast Guard type of proper size can operate for many months or even several years. These batteries are considered so dependable that they frequently are used as a standby for commercial power.

At present it is customary to operate the lights and small sound signals on offshore platforms by power obtained from such batteries. A battery system of this sort would be ideal as a power supply for the two-mile fog horn signals except for the fact that the cost of power -from such batteries to replace the power normally expended by a diesel engine sound warning system with an equivalent amount of power from such batteries is prohibitive. Such great expenditures for batteries is impractical and the industry has continued to use diesel engine power for fog horns having a two-mile audibility ice range although it is well known that battery power would greatly simplify maintenance problems and increase signal reliability.

To reduce the cost of battery power to a reasonable amount, the eiciency of power use in the sound signal system must be increased enormously from that of the present systems. Such great increase of eiciency has hitherto been believed to be impossible so that the diesel powered systems have remained in use.

It is an object of this invention to provide a batterypowered sound warning signal for offshore use in which the power required is only about J/ of that required in existing diesel powered systems.

Another object is to provide a sound warning system of this type which is suitable for service in remote and isolated locations and which may be constructed to comply with requirements of federal laws concerning sound warning systems on obstructions in navigable waters.

Another object is to provide such sound warning system which may be both installed and operated at lower costs than present diesel powered systems.

Still another object is to provide such sound warning system which consumes power in substantial amount only while the sound signal is being produced.

Another object is to provide a sound warning system of this type which requires service only at very long intervals.

Another object is to provide a sound warning system in which sound of selected wave length or frequency is concentrated in horizontal direction and in which a highly etiicient power supply converts direct current `into alternating current having the frequency of the desired sound waves.

Another object is to provide a sound warning system having no moving parts in the power supply.

Another object is to provide a sound warning system in which power is supplied to sound generators at substantially constant voltage while a battery supplying power gradually runs down and voltage from the battery decreases.

Another object is to provide a sound warning system having a power supply arranged for on-off coding and in which full power is supplied to sound generators instantaneously on operation of a coding switch.

Other objects, advantages and features of this invention will be apparent to one skilled in the art upon a consideration of the written specification, the attached claims and the annexed drawings.

The improved battery-powered sound warning system of the present invention includes a vertical array of electric sound signal generators connected to emit sound waves exactly in phase with each other at a frequency selected to give maximum audibility at a specified distance. These sound generators are driven by alternating current supplied by a circuit arranged to substantially eliminate loss of power in produ-ction of overtones, and in which voltage is maintainedat a selected value as the batteries age. A transistor-controlled power inversion and amplifying system provides for instantaneous application of full power to the sound generators and instantaneous switching to insignificant power consumption in response to operation of a small dependable coding switch.

The system of the present invention includes a battery-powered means for generating voltage oscillations of square wave type at an exact selected frequency in an output therefrom. Preferably, this frequency generator comprises a conventional battery-powered electric circuit arranged to generate sinusoidal voltage oscillations therein in response to vibrations of a self-excited tuning fork magnetically coupled with the circuit, a saturable amplier cooperative with the'circuit to amplify the volt'- age oscillations and to slightly distort the same in the direction of square wave form, a Schmitt trigger, arranged to convert the amplified and distorted sinusoidal voltage oscillations into square wave form, and a divider cooperable therewith, arranged to reduce frequency of an input signal to a sub-multiple thereof in the output. Frequency generators of this type are old in the counter art and may be purchased assembled as a unit from manufacturers of counting and computing equipment. Other types of battery-powered generators which emit square waves at an exact selected frequency can be used, but the generator described above is preferred because of its low cost, efiiciency and ready availability.

The tuning fork used should have a vibration frequency which is an exact multiple of, preferably 2 or 4 times, the frequency of sound waves desired in the sound signal to be produced. The audibility of sound at long distances from the point of its generation is usually best when the frequency of the sound waves is in the range from about 1,00 to 500 cycles per second. Within this range, some preferred frequency or range of frequencies will be found which gives maximum audibility at a specific distance from the source of sound when the er1- ergy used in producing the sound waves is constant. For example, at a distance of two miles from a source emitting sound waves at constant consumption of energy, frequencies in the range from 250 to 350 cycles per second have been found most audible.

A small tuning fork having a vibration frequency which is a multiple of the desired sound wave frequency is preferred since large, heavy tuning forks having low frequencies are difficult to tune to an exact frequency and also require considerable power to maintain vibration. Thus, if a sound having a frequency of 300 cycles per second is chosen as a signal to have maximum audibility at a range of two miles, it is preferred that the tuning fork have a vibration of 600 cycles per second. Forks having a frequency near this value are easily obtainable upon the market, consume little power and can be tuned to such exact vibration frequency that their maximum variability is about 0.001 `cycle per second. The fork used must have an exact frequency since the exact frequency of the square waves in the output from the frequency generator is important as will be seen from the description of the sound-emitting system described below.

The output from the frequency generator means carries only a minute current having voltage oscillations of small range. The output is suitably coupled to an amplifying means which serves to increase the power of the signals to a level suitable for driving the switching transistors in a power stage. A coding circuit, or preferably a coding switch, isdisposed to short-circuit, or open, or to effectively cancel the signals in the output from the frequency generator means.

Location of a coding switch in this output allows the use of a very small switch which may interrupt or pass signals from the frequency generating means to an amplifier stage at predetermined intervals with the expenditure of very little power, and permits the use of a cheap, dependable switch in a location where it is not affected by arcing. A switch driven by a rotating member carrying a cam is the preferred means for interrupting or passing signals to the amplifier stage. A switch of this type may be operated by a small, constant-speed motor whose power requirements `are so small that they are insignificant, and may be arranged to open and close the circuit, or to short-circuit the signals as desired.

While it is possible to use a single-stage amplifier power stage means, the preferred arrangement is a twostage push-pull transistor-controlled amplifying system, the two-stages being transformer coupled to each other, to the output of the frequency generator means and to a voltage regulator means to be described later. Each of the coils of each of the push-pull amplifiers is center tapped and the taps are suitably connected to opposite poles of a battery in each stage. The transistors in each of the amplifying stages are connected in the usual manner, conventional in push-pull transistor amplifiers. Push-pull amplifiers are preferred because there is sub' stantially no distortion of square waves amplified therein.

The square wave voltage form is quite important in this invention. The transistors must switch from open to full conduction, or closed condition, quickly and completely. This change in condition is accomplished by some lost power which is at a minimum when the voltage is of square wave form. The power loss across the transistors is equal to the current times the voltage drop and with square wave switching the voltage drop is zero, or nearly zero, when the current is maximum, and the current is zero or nearly zero when the voltage across the transistor is maximum. For both conditions the power loss in the transistors is zero or nearly zero. Thus, it is possible to keep the power loss in the amplifier and inverter to a very small percent of the power supplied when the voltage oscillations are of square wave type.

A transistorized amplifying system also is preferred oecause of instantaneous response to input signals, and the transistor leakage current is very low when the coding switch interrupts signals from the square wave frequency generator so that there is very little loss of current and power during the time the amplifying system is in the off part of its cycle. Since the off period of a signal often is ten times the period in which sound is being produced, it is important that power loss during this period be at a minimum.

Power from the power stage is fed to an output voltage regulator means which is a continuously variable transformer. This transformer is adjustable to maintain the proper input voltage to the sound generators. Automatic adjustment of this voltage is essential to long continued use of the device in unattended locations. The battery voltage is high when batteries are new but gradually decreases over a long period of time as the batteries are used so that an uncontrolled output voltage will result in passing current at too high voltage to the power generators and excessive consumption of power when batteries are new. Then, as the batteries gradually lose their Voltage due to high output, the current supplied to the power generators will be at a Voltage too low to give sufficient power for the range requirements.

Automatic operation of the voltage regulator preferably is accomplished by using a pair of voltage responsive relays, including a relay sensitive to high voltage and one sensitive to low voltage, to operate a positioning motor driving a 4movable contact member making connections to the secondary coil of the variable transformer in a series of positions.

Diaphragms in the sound generators may be of the tuned or untuned type. In tuned sound generators sound is produced by vibration of a diaphragm which is responsive to oscillations in an electric current flowing through a coil wound upon a soft iron core. Diaphragms of tuned type are carefully tuned to require a minimum of energy at the tuned frequency to begin and maintain oscillation of the diaphragm. Diaphragms of this type are used with coils characterized by a very high Q, that is, a very large value of inductive reactance, compared to resistance. As a result, the tuned transmitter circuit offers a high impedance to all but the fundamental sine wave frequency component of the driving square wave voltage. The other components of a square wave, that is, the harmonics of three, five, seven and higher frequency, are rejected. Hence, the signal absorbs only periodic energy at the fundamental frequency. The effect is that current in the transmitter circuit and in the battery is sinusoidal in nature.

A power factor correcting capacitor is used to reduce the driving voltage required. If, however, the exact correction is not accomplished, the reactive power is returned to the battery through a pair of rectiers in the power stage.

Untuned power sound generators also may be used if the untuned generators are supplied with current passed through a high Q electrical lter. A lter of this type is essential with untuned transmitters since, if the 11ntuned transmitters are used without the filter, the three, five, seven and higher odd harmonic frequencies present in the square driving wave will cause sound waves of corresponding high frequency, and result in waste of power in producing sound waves of short range audibility. It is preferred that a separate power stage, voltage regulator and power factor correcting capacitor, or tuned lter, be used for each sound generator in the vertical array, all controlled by the same frequency generator through a common driver stage. This preferred arrangement results in equal power consumption by each of the sound generators. While a single power stage, voltage regulator and power factor correcting capacitor can be used, it has been found that the resistances of the individual 1sound generators are somewhat unequal so that power consumption varies from generator to generator.

The inductances of tuned diaphragm generators are also somewhat unequal. Inductance in sound generators of this type is dynamic rather than constant and depends largely upon the position and thickness of the diaphragm as well a-s on the inductance of the driving coils. If the sound generators are connected in parallel to common leads containing a power factor correcting capacitance unequal inductance in the various sound generators will result in sound waves sufficiently out of phase with each other to decrease the efficiency of lateral sound lens production. The sound generators can be connected in series, and while series connection results in elimination of differences in phase, small differences in resistance still exist so that power consumption by the individual generators is unequal.

Thus, it will be seen that the present invention makes use of a combination of elements which results in achievement of the objects set forth above. A frequency generator which produces voltage oscillations of the exact frequency required for maximum audibility of sound waves ata minute expenditure of power is combined with a coding switch which passes or interrupts signals in the output. When the signals are passed, they are amplitied `in square wave form in a transistorized amplifier or driver stage and the power output from this stage is large enough to drive transistor switches in a power stage. Power at regulated voltage is then freed from the odd harmonics present in a square wave and the resulting fundamental frequency drives a diaphragm to produce sounds of a selected frequency to be most audible at a given distance. The sound generators themselves yare arranged in vertical array and the diaphragms in each of the generators vibrate in phase, thus producing the desired signals at a number of vertically spaced points so that resulting waves traveling outward therefrom leave each of the points exactly in phase and reinforce each other in a horizontal plane, thus enabling substantially all power expended in their production to be concentrated in the horizontal plane.

Details of this invention will be best understood from consideration of the following description and the attached drawings wherein like reference numerals are used to designate like parts in all figures:

FIG. 1 is a schematic representation of the system of the present invention showing conversion of sound waves from each of a` vertical series of sound generators at a distant point;

FIG. 2 illustrates a sound lens showing distribution of energy in converging sound waves from a vertical series of sound generators;

FIG. 3 illustrates distribution of energy in sound generated at a single source;

FIGS. 2 and 3 together illustrate the improvement in carrying power obtained by arranging a plurality of sound generators in a vertical array, as compared to a single point generation of sound waves using substantially the same energy as that distributed among the plurality of generators of FIG. 1;

FIG. 4 is a more detailed schematic representation of a preferred embodiment of this invention employ-ing a tuned sound generator;

FIG. 5 illustrates a preferred tuned sound generator of double type connected in a circuit effective for tuned generators;

FIG. `6 illustrates schematically a preferred arrangement 'of tuned sound generators in vertical array each having a separate power stage control-led by a common frequency generator and amplifier for driver stage, and each having separate voltage regulators and power factor correction capacitors; and

FIG. 7 illustrates a preferred sound generator :of untuned type connected in a circuit having a tuned filter therein.

In FIG. 1, the reference numeral I1 indicates generally a frequency generator to be described in detail later coupled to a driver stage 2 which is in turn coupled to a power stage 3. An output regulator 4 is disposed to regulate voltage and current from the power `stage to a plurality of sound `generator-s 5 disposed in verical array on axis Y. (These generators are illustrated as of the preferred type shown in greater detail in FIG. 5. Sound waves are emitted from all Iround generators 5 exactly in phase with each other in response to voltage oscillations in :a circuit illustrated lby a single lead 7 from the output regulator and connected to each of the generators 5. Sound waves generated lin phase with each lother traveling along path y6a to 6]', inclusive, travel along substantially equal paths and meet at a point D .Lmany wave lengths from the vertical Y axis and since these Waves are in phase, they reinforce each other when they converge. When the spacing between sound generators is equal, as illustrated, the point D will lie in a thorizontal axis XX Ib-isecting tlhe center of the array on the vertical ax-is YY. iIt can also he shovm mathematically that at any point -E spaced from the XX axis wave pressures are suhtractive so that energy in radiating sound Waves is effectively concentrated in the XX plane as 'shown fby the energy pattern in FIG. 2.

It is understood that the point l) of FIG. 1 is a point on a circle defining the maximum extent of the sound lens shown in FIG. 2 and 4llas corresponding points D1 defining the circumference of this circle in the XX plane. The vertical array of a plurality of sound generating means is therefore lvery effective in concentrating the energy on the sound signal in a horizontal lens kof sound which is audible at far greater distances than s'ound waves of the same total energy produced at a single point.

It fis preferred to use a rather large number of Isound generators, illustrated in FIG. 1 at 5, since the greater the number lof sound sources and the larger their spacing the more effective the lens is at great distances. Limitation on the number of sound sources and spacing are imposed Iby the practical limits of size and c'ost tof equipment; however, about eight to ten sound sources can usually he used Without increasing the cost and size too much.

FIG. 3 illustrates the sound radiation pattern from a single point source 8. 'It will be seen that this radiation pattern is spherical and in a patternV of this type the sound waves d'o not reinforce each other so that the energy 'is quickly dissipated and -lost in the atmosphere.

In o-rder for sound to fbe effective in fthe formation of the flattened sound lens, all Iwaves must fbe emitted exactly in phase. FIG. 4 illustrates schematically how sound Iwaves of frequency selected for maximum audibility are emitted in phase with each other in this embodiment of the invention.

'Ilhe battery-powered frequency generator f1 preferably draws its power through leads 9 and 10 from a fbattery C111 in the power stage, which also is the principal power supply for the sound generators. The preferred generator illustrated includes an electric circuit 112 arranged to have sinusoidal voltage oscillations induced therein lby a selfexcited tuning fork l'13. Circuit `12 in-cludes a driving coil '-14 and a signal coil 15 arranged in conventional manner to drive the fork and induce voltage oscillations in the circuit. Voltage wave forms 16, 17 and (18 are illustrated albove the output from each of the units making up this frequency generator and similar wave forms are shown over the `output from each unit in the system. It `will Ibe understood that the square waves shown in the drawingy have lbeen idealized in that the actual wave forms produced are not perfectly square as shown hut are substantially square and when the term square wave is used herein, it is meant that the wave is substantially square. The output from the tuning fork circuit is coupled to an amplifier 19 which preferaibly is of saturable type so that the voltage 'waves in the output from the amplifier have the same frequency as the sinusoidal waves 16 from the tuning fork circuit, but are amplified and slightly distorted into the shape shown at 17. A Schmitt trigger is coupled to the output from the amplifier and 'serves as a means for converting the distorted sinusoidal waves into square waves `having the same frequency as that of the tuning fork circuit. A divider 21, .preferably a 'binary lor quarternary divider, is coupled to the output from the Schmitt trigger and serves as a means for reducing the frequency of the square waves to one Ahalf or one fourth of the frequency in the trigger output.

A resistance illustrated as resistances 212 and 22a is connected in the output from the divider so that current llow in this circuit is very minute. A coding switch 23 is connected to interrupt or pass signals from the output from the frequency generator at predetermined intervals. A preferred device for operating this coding switch is a [rotary member 24 driven lay a small constant .speed motor (not shown) and `carrying a cam 25 in position to open and close switch 123. Switch Z3 is illustrated as connected to short-circuit the signal from the frequency generator to the primary coil 126 of a push-pull amplifier designated generally as 2 transformer coupled therewith.

This lirst -stage amplifier is transistor controlled and may include a fbattery 27 having its opposite terminals connected to center taps in secondary, or driven, coil 28 and to primary, or driving, coil 219, or preferably the center taps may lbe connected to opposite terminals of 'battery 11 in the power inverter, as illustrated. The ends of the driven coil 28 are connected to lbases of the pair of transistors 30 and 31, while the ends of the primary, or driving, coil 29 are connected to collectors of the corresponding transistors. The positive terminal of Ibattery 27 also is connected to the emitters of transistors 30 and 31 in conventional lmanner.

Thus it will be seen that, when coding switch 23 is open, signals from the frequency generator pass through the driving coil 26 and both current flow and voltage oscillations are amplified in the driver stage 2 `and are passed on in the output therefrom to the power stage.

The second stage amplifier of the power stage 3 functions principally as a power amplifier means and is constructed essentially the same as the amplifier of driver stage 2 and controls the direct current flow from the main battery 11 of the supply system to be an alternating current of square wave form. rl`hus the power amplifier means includes a controlled transistor switch means comprising transistors 35 and 36. This switch means is controlled by a fixed frequency square wave signal from a fixed frequency square wave generating means shown as the .combination of frequency generator 1 and driver stage 2 and is coupled thereto by a coupling means shown as transformer 35a. As a result, the switch means is controlled to produce a given ON-OFF period whereby fluctuating current flow to the coil of the sound generator is produced.

The power stage 3 differs principally from amplifier 2 in that it includes a pair of rectiers 32 and 33, preferably of silicon type, disposed to provide a return path for reactive power from the ends of primary, or driving, coil 34 to the battery at points between the ends of the coil and transistors 35 and 35. The reason for this arrangement will be described later.

The driving coil 34 of the power stage induces a current in coil 37 and in a primary coil of a variable transformer 44 in the output regulator designated generally as 4. The output regulator preferably includes a pair of: relays indicated generally at 38, one of said relays sensitive to low voltage and the other to high voltage and connected to supply current to the positioning motor 39 suitably connected mechanically, as indicated by the broken line 60, to move a movable contact 40 through a series of positions making contact with the secondary coil of transformer 44 at various positions along the length of the coil. Voltage regulators of this type are old in the art and are widely sold under the trade name Variacf Output current from the variable transformer 44 is connected by leads 41 and 42 to a sound generator 5, illustrated as of tuned diaphragm type. A capacitor 43 is arranged in lead 41 and cooperates with the driving coil of the sound generator 5 to reject the higher frequency components of square voltage waves such as the 3rd, 5th and 7th harmonics so that the sound generator 5 utilizes only energy at the fundamental frequency of the square Waves in the output from the power stage. Capacitor 43 has hereinbefore been referred to as a power factor correcting capacitor used to reduce the driving voltage required.

Thus it will be seen that a means is provided for coupling the output of the power amplifier means to the coil of the sound generator 5.

The rectifiers or diodes 32 and 33 provide a return path to the battery for reactive power. If this path were not provided there would be flow of current across the transistors at the time the transistors switch from closed to open condition to interrupt current into he reactive network. This would result in destruction of the transistors by the high voltage developed.

The tuned sound generator 5 is shown in greater detail in FIG. 5. In the double generator shown, a single pair of leads 41 and 42 are connected to coils 53 and 54 energizing a common soft iron core disposed to vibrate matching tuned diaphragms 51 and 52 simultaneously and in phase with each other. Preferably, a pair of double horns 55 and 56 is connected to the body of the sound generator 5 to deflect sound waves produced by each diaphragm outward.

The preferred arrangement illustrated in FIG. 6 uses a single frequency generator constructed according to FIG. 4. A single driver stage 2 is connected to amplify the output from this generator while individual power stages 3a, 3b, 3c, and 3d are provided for each of the sound generators shown as double generators 5a, 5b, 5c and 5d, each connected to the corresponding power stage by a circuit which includes individual voltage regulators 4a, 4b, 4c and 4d and capacitors 43a, 43b, 43o` and 43d. The arrangement is otherwise the same as illustrated in FIG. 4. It is to be understood that each of the power stages 3a, 3b, 3c and 3d may be supplied by separate batteries as is shown in FIG. 4 or, if preferred, may be connected to a single large battery or to two or more batteries arranged in parallel. The number of batteries used will be dictated by cost and convenience rather than by any preferred type of electrical connection. The use of individual power stages, voltage regulators and Vpower factor correction capacitors for each of the sound generators perimts all the sound generators to operate in phase with each other while being driven at the same energy output.

Untuned sound generators, preferably of the type illustrated in FIG. 7, may be substituted for the tuned generators illustrated in FIG. 6 if desired and these may be either of double diaphragm type corresponding to the generator shown in FIG. or of the single diaphragm type shown in FIG. 7. The untuned sound generators differ, however, from the tuned type in that they contain a permanent magnet 61 and that diaphragm 62 is not tuned to require a minimum of energy to begin and maintain oscillation at any particular frequency. The coil 63 driving diaphragm 62 is wound upon a suitable cylinder 64 of any light weight insulating material attached to diaphragm 62 so that as current oscillations occur in the coil 63, the coil and diaphragm are drawn toward and repelled from permanent magnet 61 in response thereto.

Since coils 63 used in sound generators of this type are not of the high Q type used with tuned generators, when untuned generators are used it is necessary to include a tuned filter in the leads to the sound generator. The operation is otherwise the same as described under the tuned generator of FIG. 5

From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the system.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

The invention having been described what is claimed is:

ll. A sound system comprising, in combination, a sound generator having a diaphragm and an electrical coil arranged to vibrate the diaphragm upon supplying a fluctuating current flow to the coil; a battery; means converting and presenting power of the battery to said sound generator including a power amplifier means including a controlled transistor switch means, a fixed frequency substantially square wave generating means for controlling said switch means and means coupling said square wave generating means to said switch means to control said switch means to produce a given ON-OFF period whereby the said uctuating current ow to said coil is produced; and means coupling the output of said power amplifier means to the coil of said sound generator.

2. The system of claim 1 wherein the last mentioned means includes inductive and capacitive components tuned in series-resonance to a fundamental frequency equal to the frequency at which said diaphragm is to be vibrated whereby current fluctuations corresponding to harmonics of said fundamental frequency are substantially prevented from flowing through the coil.

3. The system of claim 1, wherein said diaphragm is tuned to vibrate at a preselected fundamental frequency and wherein said coil has a capacitor connected in seriesresonance with it and also a suflciently high Q so that it and the capacitor present a high impedance to the harmonic components of the output of said power amplifier means.

4. The system of claim 3 wherein diode means are connected in shunt across said switch means to conduct reactive power back to the battery.

5. The system of claim 1 wherein said power amplifier means is a push-pull amplifier.

6. A sound warning system comprising, in combination, a plurality of sound generators in vertical array with each having a diaphragm and an electrical coil arranged to vibrate the diaphragm upon supplying a fluctuating current flow to the coil; at least one battery; means converting and presenting power of the battery to said sound generators including a power amplifier means for each sound generator, each of said power amplifier means including a controlled transistor switch means, a single fixed frequency substantially square wave generating means for controlling all of said switch means and means coupling said square wave generating means to each of said switch means to control said switch means to produce a given ON-OFF period whereby the said fluctuating current flow to each of said coils is produced; and means coupling the output of said power amplifier means to the respective coils of said sound generators, whereby each of said sound generators will vibrate at the same frequency.

7. The system of claim 6 wherein said diaphragms are tuned so that all vibrate at the same preselected fundamental frequency and wherein said coils each have a capacitor connected in series-resonance with it and each also has a sufficiently high Q so that with its respective capacitor, the coil and capacitor present a high impedance to the harmonic components of the output of said power amplifier means.

References Cited by the Examiner UNITED STATES PATENTS 1,721,865 7/39 Johnson et al SLi-1.11 2,455,472 12/48 Curl et al. 340-384 2,620,390 12/ 52 Sunderland 340-25 2,790,164 8/57 Oberg 340-388 2,910,689 10/59 Grace 340-384 2,948,784 8/60 Yarger 200-28 NEIL C. READ, Primary Examiner. 

1. A SOUND SYSTEM COMPRISING, IN COMBINATION, A SOUND GENERATOR HAVING A DIAPHRAGM AND AN ELECTRICAL COIL ARRANGED TO VIBRATE THE DIAPHRAGM UPON SUPPLYING A FLUCTUATING CURRENT FLOW TO THE COIL; A BATTERY; MEANS CONVERTING AND PRESENTING POWER OF THE BATTERY TO SAID SOUND GENERATOR INCLUDING A POWER AMPLIFIER MEANS INCLUDING A CONTROLLED TRANSISTOR SWITCH MEANS, A FIXED FREQUENCY SUBSTANTIALLY SQUARE WAVE GENERATING MEANS FOR CONTROLLING SAID SWITCH MEANS AND MEANS COUPLING SAID SQUARE WAVE GENERATING MEANS TO SAID SWITCH MEANS TO CONTROL SAID SWITCH MEANS TO PRODUCE A GIVEN ON-OFF PERIOD WHEREBY THE SAID FLUCTUATING CURRENT FLOW TO SAID COIL IS PRODUCED; AND MEANS COUPLING THE OUTPUT OF SAID POWER AMPLIFIER MEANS TO THE COIL OF SAID SOUND GENERATOR. 